The present invention relates to a circuit arrangement for generating a control voltage which is dependent upon an alternating voltage.
Such circuit arrangements are disclosed in applicant's U.S. Pat. Nos. 4,321,482 and 4,322,636. It is possible to use these circuit arrangements as control voltage generators in compander systems as disclosed, for example, in U.S. Pat. No. 3,969,680 issued to Jurgen Wermuth.
In such a compander circuit, the input of the control voltage generator is connected in a useful signal path containing an electronically gain controlled amplifier and receives the alternating voltage output signal of that gain controlled amplifier for compression operation or the alternating voltage input signal of such gain controlled amplifier for expansion operation. The direct voltage output signal from the control voltage generator is fed to the control input of the amplifier disposed in the useful signal path.
The control voltage generator here acts in such a manner that, if the alternating voltage fed to the input of the control voltage generator increases rapidly, the control voltage generator generates a direct voltage which rapidly changes in magnitude and which quickly changes the gain of the amplifier disposed in the useful signal path. A rapid change in the gain of the gain controlled amplifier disposed in the useful signal path is of particular importance if the level of the useful signal suddenly changes over a wide range. Otherwise, if the signal level jumps from a low to a high value there exists the danger of the compressor output signal overshooting and thus the transmission channel being overdriven. With a level jump in the opposite direction, noise signals would become audible at the output of the expander if the output of the control voltage generator did not change fast enough. While the useful signal was still present, these noise signals would be covered by it. To realize complementary behavior of compressor and expander, it is advisable to consider the features specific for compressor and expander operation with equal weight for both compressor and expander.
The control voltage generator for controlling the gain of the amplifier disposed in the useful signal path must therefore furnish a control voltage which is quickly adapted to the present level of the useful signal. A pre-requisite for this is a short discharge time constant at the charging capacitor of the control voltage generator which becomes effective when there are large jumps in level. In the stationary state or for slow changes in signal level, however, the discharge time constant must be large so that the control voltage does not fluctuate in the rhythm of the useful signal frequency. Such behavior would result in a high distortion factor.
When, in this connection, the terms discharge or charge change are used, it is to be understood that the above considerations apply to circuits in which the charging capacitor is discharged with decreased alternating voltage amplitude as well as to those in which it is charged.
In a determination of the discharging time constant for the charging capacitor, several mutually contradictory requirements must be met so that a circuit arrangement could be provided which controlled the discharging time constant in a manner to provide very reliable protection against overshooting, a low distortion factor and the physiological covering, or masking, effect of noise signals by strong useful signals.
One such circuit arrangement is disclosed in my U.S. Pat. No. 4,318,009. This known solution has the advantage that the delay period of the delay member can be controlled with respect to a short duration of unmasked noise while maintaining a permissible distortion factor.
My U.S. Pat. No. 4,270,103 discloses a rectifier circuit for the compander process disclosed in my U.S. Pat. No. 4,318,009 and which provides sections of linear characteristics by limiting the rectifier voltage.